The invention disclosed and claimed herein generally pertains to passive heat transfer apparatus for an X-ray imaging system having a rotating anode X-ray tube, wherein the heat transfer apparatus is disposed to conduct heat away from regions proximate to the tube. More particularly, the invention pertains to apparatus of the above type wherein the head of an imaging subject is typically positioned so that noises generated proximate to an X-ray tube are particularly disturbing. Even more particularly, the invention pertains to apparatus of the above type which is very useful in connection with X-ray systems used for mammography, and which provides means for reducing acoustic disturbance.
In a rotating anode X-ray tube, a beam of electrons is directed through a vacuum, across very high voltage, from a cathode to a focal spot position on an anode. X-rays are produced as electrons strike the anode, comprising a refractory metal track, such as tungsten, molybdenum or rhodium, which is rotated at high speed. However, the conversion efficiency of X-ray tubes is quite low, typically less than 1% of the total power input. The remainder, in excess of 99% of the input electron beam power, is converted to thermal energy or heat. Accordingly, heat removal, or other effective procedure for managing heat, tends to be a major concern in the design and operation of an X-ray tube. Frequently, fans or the like are employed to circulate air to cool the tube.
In an X-ray imaging system designed for mammography, the patient is usually positioned so that her ears are very close to the X-ray tube, that is, within two or three centimeters. Typically, two significant sources of noise are located proximate to the tube. One source is the bearings contained within the tube casing, to support the rotary anode. The bearings produce an unpleasant high frequency noise as the anode rotates during X-ray generation. The other noise source is an arrangement of fans, which are typically located in a housing which also contains the tube, the fans being operated to circulate a stream of cooling air around the tube. Noise generated by both sources tends to be very disturbing to a mammography patient.
In the past, efforts have been made to reduce noise levels by surrounding the X-ray tube and the fans with sound absorbing material. However, materials commonly used for this purpose also tend to be thermally insulating. Thus, this approach to solving the noise problem prevents dissipation of heat away from the tube, so that the temperature of the tube may be quickly driven above the tube temperature limit.
The invention is directed to passive heat transfer apparatus for an X-ray imaging system provided with a rotating anode X-ray tube, wherein the heat transfer apparatus is disposed to rapidly conduct heat away from the tube and dissipate it into the surrounding environment. The apparatus may also be adapted to provide acoustic damping, or to reduce noise levels, and is particularly well suited for use in connection with X-ray equipment designed for mammography applications. However, the invention is by no means limited thereto. The invention generally comprises a thermally conductive plate or other support member, which is located in an X-ray tube housing in spaced apart relationship with the casing of the X-ray tube, also located in the housing. The invention further comprises an elongated heat transfer device having a first end which is proximate to the tube casing and a second end which is proximate to the support plate. A quantity of selected working fluid is sealably contained in the heat transfer device, the working fluid being disposed for bi-directional movement along the device to transfer heat from the first end of the transfer device to the second end thereof. Thus, the heat transfer device is passive and comprises a convective device, that is, employs fluid to move heat along its length. A heat dissipation device is provided to extend through the housing proximate to the support plate, for transferring heat from the support plate to air external to and surrounding the housing.
In a preferred embodiment of the invention, the heat transfer device comprises a conduit segment of selected length, the conduit segment having an inner wall in adjacent relationship with a sealed interior space. A selected porous material is attached to the inner wall and configured to define a passage through the sealed interior space that extends along the length of the conduit segment, the porous material being selected in relation to the working fluid so that the fluid, when in liquid form, is disposed for movement through the porous material by means of capillary action. When the first end of the transfer device is at a selectively higher temperature than the second end, the fluid proximate to the first end is vaporized into gaseous form, moved along the passage by means of convection to the second end, and then condensed into liquid form. Preferably, a layer of sound absorbing material is placed around the X-ray tube within the housing, to serve as a barrier to noise generated by anode rotation within the tube. Preferably also, the heat dissipation device comprises a number of cooling fins which are thermally joined to the support plate, and extend through the wall of the housing into the surrounding air or environment.